(1) Field of the Invention
This invention relates generally to systems and methods for forming metal oxide and metal-based oxide alloy films for semiconductor applications, and, more particularly, to a hybrid beam deposition (HBD) system and methods for fabricating metal oxide ZnO films, p-type ZnO films, and ZnO-based II-VI group compound semiconductor devices such as light-emitting diodes (LEDs), laser diodes (LDs), photodetectors, and gas sensors and bipolar/unipolar semiconductor devices such as heterojunction bipolar transistors (HBT) and field-effect transistors (FET)s.
(2) Description of Related Art
Zinc oxide (ZnO) films have been extensively studied for use in piezoelectric and waveguide devices. GaN is known in the prior art as a good material for the fabrication of optical devices such as blue light emitting devices (blue LEDs) and blue laser diodes (blue LDs). GaN has been utilized in an effort to increase the average lifetime of blue LDs up to 10,000 hours at room temperature. ZnO thin films are possible substitutes for gallium nitride (GaN) film applications because the optical properties of ZnO films are very similar to those of GaN and both materials possess the same crystal structure, i.e., wurtzite, with small lattice mismatch.
ZnO films represent potential candidates for short-wavelength (ultraviolet/violet/blue) optoelectrical and optical applications based upon similarities in crystal structure and optical properties between ZnO films and GaN films. ZnO has physical characteristics that make it a more attractive candidate than GaN for such applications. For example, high quality ZnO films having very low defect densities can be synthesized, high-quality ZnO substrates are available for homo-epitaxial growth, ZnO films provide an emission source for very pure monochromatic light, and ZnO, which is a wide band-gap semiconductor material (Eg of approximately 3.4 eV at room temperature), is a relatively hard material. The strength of the Zn-to-O bond of ZnO is greater than the Ga-to-N bond of GaN. This difference in bonding strength can lead to significantly different results for p-type doping. ZnO has a melting temperature of approximately 2000° C. (versus a melting temperature of approximately 1700° C. for GaN). Thus, ZnO is sufficiently stable with respect to the high temperature annealing and treatment processes associated with doping and the formation of ohmic contacts as well as device fabrication. Other attractive characteristics of ZnO thin films include: very strong spontaneous and stimulated emissions by excitons (even at room temperature); and the ready availability of large-area ZnO substrates.
The practical development of semiconductor devices based upon ZnO films requires the ability to consistently and expeditiously fabricate both n-type and p-type ZnO films of high quality. A variety of techniques are known in the prior art for fabricating or synthesizing thin films for different applications, including semiconductor applications. Such techniques include chemical vapor deposition (CVD), molecular beam epitaxy (MBE), and pulsed laser deposition (PLD). Specialized variations of these techniques, e.g., electron cyclotron resonance chemical vapor deposition (ECR-CVD), metal organic chemical vapor deposition (MOCVD), solid-source or gas-source MBE (SMBE or GMBE), which emphasize the physical aspects of the deposition-material sources, are also known in the art for fabricating or synthesizing thin films.
Although PLD is one of the best growth methods for oxide materials, it needs to be improved for synthesizing high-quality ZnO films. For example, a target for laser ablation should provide uniformity in mixing dopant material with pure ZnO material for the synthesis of uniformly doped ZnO films. The mixture ratio of dopant material to host ZnO material in such a target should simultaneously be very adaptable to obtain any level of the carrier density required for semiconductor device applications. Unfortunately, it is very difficult to prepare a target satisfying these conditions.
Among the foregoing techniques, the MBE technique is very useful for synthesizing high-quality thin films and fabricating quantum-well devices, especially when the deposition-material sources have low melting temperatures and high evaporation pressures such that crystal formation can be achieved under low temperature and low pressure conditions. For example, GaAs and zinc selenide (ZnSe) films are suitable candidates for synthesis using MBE. One conventional MBE technique utilizes separated sources comprising a pure zinc molecular beam and a pure oxygen molecular beam to synthesize ZnO films. The effective pressure ratio of the oxygen to zinc constituents is a critical constraint for synthesizing high-quality ZnO films using this MBE technique. The oxygen constituent should be over pressured to avoid any oxygen vacancies. Over pressurization of the oxygen constituent, however, in this MBE technique is limited due to the adverse effects of oxidation on the MBE equipment, which limits the maximum rate of high-quality ZnO film growth. Conventional CVD techniques are also subject to such oxidation limitations.
It is generally known that the crystal qualities for SiC and GaN films grown at a high temperature and in a high pressure would be better than those for SiC and GaN films grown at a low temperature and in a low pressure. The growth of SiC and GaN films at a high temperature and in a high pressure is performed by using the MOCVD technique. For these reasons, the MOCVD technique becomes a common technique to fabricate short-wavelength light emitting devices, for example, with GaN-based materials. Compared to the MOCVD technique, however, the MBE technique is not good for the growth of wide band-gap hard materials such as SiC and GaN, but it is still preferable since the process for MBE film growth is simpler, more clearly understandable, and more easily controllable than the MOCVD technique.
A need exists to provide one or more simple and efficient techniques and the concomitant equipment for synthesizing undoped (semi-insulating) and doped (both n-type and/or p-type) ZnO thin films having lower values of electrical resistivity and higher values of Hall mobility for semiconductor applications. Such techniques and equipment should facilitate synthesis of such ZnO films at relatively low oxygen pressures while concomitantly providing a flux density of available reactive oxygen that accommodates a maximum growth rate of high-quality metal oxide thin films.